Recently, many technical developments have been remarkably made in the field of audio equipment. For example, a stereophonic system has been widely used in audio equipment. Digital systems also have been widely used for processing audio signals. These systems make the reproduced sound more similar to the original sound.
Furthermore, a sound effect processing apparatus capable of producing a specific reproduced sound field suitable to a listener's preference, by processing an audio source signal, such as music signal, has been strongly demanded in recent years.
FIG. 1 shows a conventional audio signal processing apparatus for producing such a specific reproduced sound field. In FIG. 1, an audio signal input terminal 101 receives an audio signal. The audio signal is supplied from a CD (Compact Disc) player, a tape player, VTR (Video Tape Player), or a LD (Laser Disc) player, for example. The audio signal is applied to an analog to digital converter (referred to as A/D converter hereafter) 103 through a low pass filter (referred to as LPF hereafter) 102. The LPF 102 removes undesired high frequency components (referred to as HF or HF components) from the audio signal. The audio signal output from the LPF 102 is analog. The A/D converter 103 converts the analog audio signal to digital audio signal.
The digital signal is applied to a sound effect processor 104. The sound effect processor 104 produces a plurality of reverberation sound signals, e.g., two reverberation sound signals by processing the digital signal. The reverberation sound signals thus produced almost correspond to reverberation sounds in a concert hall, or other similar sound fields. The sound effect processor 104 is typically constructed of, for example, delay units, adders, multipliers and the like.
The reverberation sound signals are converted into analog reverberation sound signals by digital to analog converter (referred to as D/A converters hereafter) 105 and 106. The analog reverberation sound signals are applied to amplifiers 109 and 110 through LPFS 107 and 108. The LPFS 107 and 108 remove undesired HF components from the analog reverberation sound signals. The amplifiers 109 and 110 amplify the reverberation sound signals and then supply the signals to loudspeakers 111 and 112.
FIG. 1 shows only one channel of the audio signal processing apparatus for simplicity. However, the audio signal processing apparatus generally includes two channels for processing stereophonic signals. Then, actually four sets of the loudspeakers are arranged at the front left and right and rear left and right. Thus, the loudspeakers may produce specific sound effects for listeners according to the reverberation sound signals.
In short, in this surround system, the sound effect processor 104 performs various signal processing operations for two channel input audio signals and by outputting four channel sound, forms a sound field surrounding listeners. As a result, listeners are able to listen as if they were actually in a concert hall or a sports arena.
When creating an atmosphere equivalent to, for instance, a concert hall, the sound effect processor 104 produces reverberation sound for 1 second (sec) to 2 secs. However, this reverberation sound is produced not only for music but also when, for instance, an announcer or a master of ceremony (referred to as M.C. hereafter) is speaking. There is a problem because this reverberation sound is unnatural, and it is hard to hear what the M.C. is saying.
Further, when processing sound from a sports arena, the sound effect processor 104 produces, for instance, an echo of about several hundreds of milli-seconds (ms). This echo is produced not only for shouts of encouragement by the audience, but also is added to the voices of announcers or commentators, and the same problems mentioned above are caused.